The present invention relates to a structure of an insulated gate type field effect semiconductor device (generally referred to as a thin film transistor or TFT) having a thin film semiconductor formed on an insulating substrate, and also to a method for producing the device.
Heretofore has been known an insulated gate type field effect semiconductor device (hereinafter referred to as TFT) having a thin film semiconductor formed on an insulating substrate (especially, glass substrate). Such TFT formed on an insulating substrate is used in various devices such as liquid crystal display devices, image sensors, etc.
In the TFT, generally used is silicon dioxide (SiO.sub.2) as the gate insulating film.
It is necessary to use a crystalline silicon film as the active layer in TFT so as to obtain high level characteristics. To produce such crystalline silicon film, there are known a method of directly forming a silicon film having a microcrystalline structure by thermal CVD (chemical vapor deposition) under reduced (low) pressure or the like, a method of making an amorphous silicon film crystalline by heating it or by irradiation of laser rays thereto, etc.
In any of these methods, however, it is impossible to obtain a monocrystalline structure at present. In other words, the crystalline silicon film to be obtained by any of said methods has a polycrystalline structure or microcrystalline structure, or has a mixed structure comprising a crystalline structure and an amorphous structure, or even has a composite structure partly containing a crystalline structure.
Since the above mentioned crystalline structures have many dangling bonds of silicon, it is necessary to introduce hydrogen into the active layer so as to neutralize (or terminate) said dangling bonds. In other words, it is necessary to hydrogenate the active layer.
However, the existence of hydrogen in the gate insulating film in TFT must be evaded as much as possible. This is because mobile ions, if any, in the gate insulating film in driving TFT cause hysteresis or variation in the threshold value.
When a conventional TFT is formed on a glass substrate, the whole device is often charged with static electricity, thereby causing a problem in that the gate insulating film is damaged by dielectric breakdown due to the static electricity. More concretely, there was a problem in that a high voltage is applied to the whole device via the gate insulating film due to the charging of the device with static electricity and, as a result, the gate insulating film becomes undurable to the voltage.
The above problem is considered to be caused by the reasons that the energy band gap (Eg) in the silicon dioxide (SiO.sub.2) film is large to be about 8 eV while the relative dielectric constant of the film is relatively small to be about 3.8.
In place of the silicon dioxide film, a silicon nitride (Si.sub.3 N.sub.4) film having Eg of about 5 eV and a relative dielectric constant of about 7 as the gate insulating film may be used. However, if such silicon nitride film is used as the gate insulating film, the Si cluster is to be a charge capture center so that hysteresis occurs in the C-V (capacitance-voltage) characteristic. Also, in the B-T (bias-temperature) treatment, there occurs another disadvantage in that a threshold voltage .DELTA.V.sub.th is shifted by about 10 V. If such silicon nitride film is used as the gate insulating film, the insulating film have a charge capture center in itself. Therefore, silicon nitride is unfavorable to be formed into a gate insulating film.
The active layer is required to contain hydrogen. When the active layer contains hydrogen, however, there occurs another problem in that the hydrogen in said active layer diffuses out into the adjacent gate insulating film.
Such is contradictory to the requirement that the gate insulating film adjacent to the active layer shall not contain hydrogen.
When a material consisting essentially of metal is employed as the material of the gate electrode, there occurs still another problem in that said metal material is alloyed with the semiconductor component constituting the gate insulating film.